Caption: Staphylococcus aureus bacteria. Coloured transmission electron micrograph (TEM) of a section through Staphylococcus aureus bacteria, a common cause of skin infections in humans. These spherical bacteria (cocci) are Gram-positive. The cells at centre and at upper left are in the process of dividing. S. aureus is harmlessly present in most people, on the skin and in the mucous membranes of the nose and throat. However, infection can occur if the bacteria are trapped within a blocked sweat or sebaceous gland, or when skin becomes broken. Symptoms of infection include boils, pustules and internal abscesses. Magnification unknown.